Method and apparatus for improved burst mode during power conversion

ABSTRACT

A method and apparatus for converting DC input power to AC output power. The apparatus comprises an input capacitor, a DC-AC inverter, a burst mode controller for causing energy to be stored in the input capacitor during at least one storage period and the energy to be drawn from the input capacitor during at least one burst period, wherein the AC output power is greater than the DC input power during the at least one burst period; a first feedback loop for determining a maximum power point (MPP) and operating the DC-AC inverter proximate the MPP; and a second feedback loop for determining a difference in a first power measurement and a second power measurement, producing an error signal indicative of the difference, and coupling the error signal to the first feedback loop to adjust at least one operating parameter of the DC-AC inverter to drive toward the MPP.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.13/206,311, filed Aug. 9, 2011, U.S. Pat. No. 8,319,378, issued Nov. 27,2012, which is a continuation of U.S. patent application Ser. No.12/804,943, filed Aug. 2, 2010, U.S. Pat. No. 8,035,257, issued Oct. 11,2011, which is a continuation of U.S. Pat. No. 7,768,155, issued Aug. 3,2010. Each of the aforementioned related patents are herein incorporatedby reference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

Embodiments of the present disclosure generally relate to powerconversion and, more particularly, to a method and apparatus forimproved burst mode operation.

2. Description of the Related Art

Solar panels have historically been deployed in mostly remoteapplications, such as remote cabins in the wilderness or satellites,where commercial power was not available. Due to the high cost ofinstallation, solar panels were not an economical choice for generatingpower unless no other power options were available. However, theworldwide growth of energy demand is leading to a durable increase inenergy cost. In addition, it is now well established that the fossilenergy reserves currently being used to generate electricity are rapidlybeing depleted. These growing impediments to conventional commercialpower generation make solar panels a more attractive option to pursue.

Solar panels, or photovoltaic (PV) modules, convert energy from sunlightreceived into direct current (DC). The PV modules cannot store theelectrical energy they produce, so the energy must either be dispersedto an energy storage system, such as a battery or pumpedhydroelectricity storage, or dispersed by a load. One option to use theenergy produced is to employ one or more inverters to convert the DCcurrent into an alternating current (AC) and couple the AC current tothe commercial power grid. The power produced by such a distributedgeneration (DG) system can then be sold to the commercial power company.

PV modules have a nonlinear relationship between the current (I) andvoltage (V) that they produce. A maximum power point (MPP) on an I-Vcurve for a PV module identifies the optimal operating point of the PVmodule; when operating at this point, the PV module generates themaximum possible output power for a given temperature and solarirradiance. Therefore, in order to optimize power drawn from a PVmodule, a power conversion device coupled to the PV module, such as aninverter or a micro-inverter, generally employs a maximum power pointtracking (MPPT) technique to ensure that the PV module is operated atthe current and voltage levels corresponding to its MPP. The MPPT actsto rapidly adjust the PV module operating current and voltage levels inresponse to changes in solar irradiance and/or temperature such that thePV module can continue to operate at the MPP.

During the time period required for an MPPT technique to bias a PVmodule to its MPP, for example, when the solar irradiance on a PV modulechanges from no irradiance to increasing irradiance or at a PVmodule/inverter initial activation, a power conversion device coupled tothe PV module will suffer from a lower efficiency until the MPP isachieved. Additionally, a power conversion device coupled to a PV modulegenerally will suffer from a lower efficiency when the PV module isoperating at a low power, e.g., low irradiance. During low irradiance, aPV module and an associated inverter may operate so inefficiently thatis it best for overall system efficiency to deactivate the PV moduleand/or its inverter until solar irradiance increases.

Therefore, there is a need in the art for a method and apparatus forimproving operation of a PV module and inverter in achieving andtracking the maximum power point.

SUMMARY OF THE INVENTION

Embodiments of the present invention generally relate to a method andapparatus for converting DC input power to AC output power. Theapparatus comprises an input capacitor, a DC-AC inverter, a burst modecontroller, a first feedback loop, and a second feedback loop. The burstmode controller causes energy to be stored in the input capacitor duringat least one storage period and the energy to be drawn from the inputcapacitor during at least one burst period, wherein the AC output poweris greater than the DC input power during the at least one burst period.The first feedback loop determines a maximum power point (MPP) andoperates the DC-AC inverter proximate the MPP. The second feedback loopdetermines a difference in a first power measurement and a second powermeasurement, produces an error signal indicative of the difference, andcouples the error signal to the first feedback loop to adjust at leastone operating parameter of the DC-AC inverter to drive toward the MPP.

BRIEF DESCRIPTION OF THE DRAWINGS

So that the manner in which the above recited features of the presentinvention can be understood in detail, a more particular description ofthe invention, briefly summarized above, may be had by reference toembodiments, some of which are illustrated in the appended drawings. Itis to be noted, however, that the appended drawings illustrate onlytypical embodiments of this invention and are therefore not to beconsidered limiting of its scope, for the invention may admit to otherequally effective embodiments.

FIG. 1 is a block diagram of a system for distributed generation (DG) inaccordance with one or more embodiments of the present invention;

FIG. 2 is a block diagram of an inverter in accordance with one or moreembodiments of the present invention;

FIG. 3 is a block diagram of a burst mode controller in accordance withone or more embodiments of the present invention;

FIG. 4 is a plurality of waveforms depicting collection periods of PVmodule power and voltage measurements during burst mode in accordancewith one or more embodiments of the present invention;

FIG. 5 is a block diagram of a DC voltage controller in accordance withone or more embodiments of the present invention;

FIG. 6 is a block diagram of an MPPT controller in accordance with oneor more embodiments of the present invention; and

FIG. 7 is a flow diagram of a method for operating an inverter in burstmode in accordance with one or more embodiments of the presentinvention.

DETAILED DESCRIPTION

FIG. 1 is a block diagram of a system 100 for distributed generation(DG) in accordance with one or more embodiments of the presentinvention. This diagram only portrays one variation of the myriad ofpossible system configurations. The present invention can function in avariety of distributed power generation environments and systems.

The system 100 comprises a plurality of inverters 102 ₁, 102 ₂ . . . 102_(n), collectively referred to as inverters 102, a plurality of PVmodules 104 ₁, 104 ₂ . . . 104 _(n), collectively referred to as PVmodules 104, an AC bus 106, and a load center 108.

Each inverter 102 ₁, 102 ₂ . . . 102 _(n) is coupled to a PV module 104₁, 104 ₂ . . . 104 _(n), respectively. In some embodiments, a DC-DCconverter may be coupled between each PV module 104 and each inverter102 (e.g., one converter per PV module 104). Alternatively, multiple PVmodules 104 may be coupled to a single inverter 102 (i.e., a centralizedinverter); in some embodiments, a DC-DC converter may be coupled betweenthe PV modules 104 and the centralized inverter.

Each inverter 102 employs an MPPT technique to operate the subtending PVmodule 104 at its MPP such that the PV module 104 generates an optimalpower output for a given temperature and solar irradiation. Theinverters 102 are coupled to the AC bus 106, which in turn is coupled tothe load center 108. The load center 108 houses connections betweenincoming power lines from a commercial power grid distribution systemand the AC bus 106. The inverters 102 convert DC power generated by thePV modules 104 into AC power, and meter out AC current that is in-phasewith the AC commercial power grid voltage. The system 100 couples thegenerated AC power to the commercial power grid via the load center 108.

In accordance with one or more embodiments of the present invention, theinverters 102 employ a “burst mode” during initial operation. In burstmode, the inverters 102 store energy over one or more AC grid voltagecycles (“energy storage periods”) and subsequently “burst” the storedenergy to the commercial power grid (“burst periods”). The length of theenergy storage periods is determined such that a ripple voltageoverriding the PV module output voltage remains below a desired ripplevoltage threshold.

In addition to improving the efficiency of the inverters 102, the burstmode facilitates a rapid convergence to the MPP utilizing an MPPTtechnique described below. Upon operating proximate the MPP, and if thePV modules 104 are supplying sufficient output power, the inverters 102deactivate the burst mode and operate in a continuous mode, utilizingthe MPPT technique to remain proximate to the MPP. In the event that thesolar irradiance and/or temperature changes to a level such that theoutput power from the PV modules 104 drops below a burst mode threshold,one or more of the inverters 102 switch back to burst mode.

FIG. 2 is a block diagram of an inverter 102 in accordance with one ormore embodiments of the present invention. The inverter 102 comprises anI-V monitoring circuit 204, an input capacitor 220, a DC-AC inverter208, a DC voltage controller 210, an MPPT controller 212, a burst modecontroller 224, and a conversion control module 214. The inverter 102 iscoupled to the PV module 104 and to the commercial power grid.

The I-V monitoring circuit 204 is coupled to the PV module 104, theinput capacitor 220, and the burst mode controller 224. The burst modecontroller 224 is further coupled to the DC voltage controller 210, theMPPT controller 212, and the conversion control module 214.Additionally, the MPPT controller 212 is coupled to the DC voltagecontroller 210, and the DC voltage controller 210 is coupled to theconversion control module 214. The DC voltage controller 210 functionsto bias the PV module 104 at a DC voltage set point (i.e., a desired PVmodule operating voltage), while the MPPT controller 212 drives such DCvoltage set point to correspond to the MPP voltage. The burst modecontroller 224 functions to switch the inverter 102 between continuousmode and burst mode, and additionally measures input power from the PVmodule 104 that is utilized by the MPPT controller 212 in determiningthe DC voltage setpoint.

The I-V monitoring circuit 204 monitors the instantaneous input voltageand current, V_(in) and I_(in), respectively, from the PV module 104.The input capacitor 220, in addition to being coupled to the I-Vmonitoring circuit 204, is coupled to the DC-AC inverter 208, and theDC-AC inverter 208 is further coupled to the DC voltage controller 210,the conversion control module 214, and the commercial power grid. Acurrent I_(cap) flows through the input capacitor 220, and a currentI_(inv) flows to the DC-AC inverter 208.

The conversion control module 214 receives a reference signal from thecommercial power grid, and provides the control signals for the DC-ACinverter 208 to convert the received DC current, I_(inv) to a requiredAC output current, I_(req). One example of such power conversion iscommonly assigned U.S. Patent Application Publication Number2007/0221267 entitled “Method and Apparatus for Converting DirectCurrent to Alternating Current” and filed Sep. 27, 2007, which is hereinincorporated in its entirety by reference. The AC output current I_(req)from the DC-AC inverter 208 is coupled to the commercial power grid suchthat it is in-phase with the commercial AC current.

The DC voltage controller 210 employs a first feedback loop (an “inner”loop) 216 to bias the PV module 104 at the DC voltage setpoint bymodulating the current I_(in) drawn from the PV module 104. The firstfeedback loop 216 comprises the I-V monitoring circuit 204, the burstmode controller 224, the MPPT controller 212, the DC voltage controller210, and the DC-AC inverter 208. The DC voltage controller 210 receivesa signal indicative of the PV module DC (i.e., average) input voltageV_(DC) from the burst mode controller 224, and receives the DC voltagesetpoint from the MPPT controller 212. Based on a difference between theV_(DC) and the DC voltage setpoint, the first feedback loop 216 drivesthe DC-AC inverter 208 to generate I_(req) such that the appropriatecurrent I_(in) is drawn from the PV module 104 to bias the PV module 104at the DC voltage setpoint. Thus, the first feedback loop 216iteratively computes a difference between the average PV moduleoperating voltage and a DC voltage setpoint for the PV module 104, andaccordingly adjusts the current I_(in) drawn from the PV module 104 suchthat the PV module 104 is biased at the DC voltage setpoint (i.e., at anoperating current and voltage that approximately corresponds to theMPP).

The MPPT controller 212 employs a second feedback loop 218 (an “outer”loop) to adjust the DC voltage setpoint such that it corresponds to theMPP voltage. The second feedback loop 218 comprises the I-V monitoringcircuit 204, the burst mode controller 224, the MPPT controller 212, andthe DC voltage controller 210. The burst mode controller 224 receivessignals indicative of the instantaneous PV module input current andvoltage, I_(in) and V_(in), respectively, through the I-V monitoringcircuit 204, and computes the instantaneous input power, P_(in), fromthe PV module 104. The burst mode controller 224 further processes theinput power P_(in), as described in greater detail below, to obtain afirst and a second power measurement and provides such powermeasurements to the MPPT controller 212. The MPPT controller 212determines a power difference between the first and second powermeasurements; based on the difference, the MPPT controller 212determines whether the PV module operating voltage must be increased ordecreased to reach the MPP, modifies the DC voltage setpointaccordingly, and supplies the new DC voltage setpoint to the DC voltagecontroller 210. Additionally, a power difference of zero indicates thatthe PV module 104 is currently biased at the MPP, and the MPPTcontroller 212 supplies the corresponding DC voltage setpoint to the DCvoltage controller 210. The second feedback loop 218 thus iterativelydetermines whether the PV module 104 is operating proximate the MPP and,in the case where the PV module 104 is not operating proximate the MPP,modifies at least one operating parameter within the first feedback loop216 to achieve the MPP (i.e., the outer loop “fine tunes” the settingestablished by the inner loop).

Upon initial operation or sufficiently low output power from the PVmodule 104, the burst mode controller 224 operates the inverter 102 inburst mode, wherein during energy storage periods (e.g., one or more ACgrid voltage cycles of 16.67 msec), the input capacitor 220 storesenergy that is subsequently supplied to the DC-AC inverter 208 during aburst period. During burst mode, the burst mode controller 224 drivesthe DC voltage controller 210 such that the output current generated bythe inverter 102, I_(req), is a “burst mode current”, I_(B). Once the PVmodule 104 is operating proximate the MPP voltage, and if the PV module104 is generating sufficient output power (i.e., if the PV module outputpower is greater than a pre-determined burst mode threshold), the burstmode controller 224 operates the inverter 102 in continuous mode. If thePV module output power subsequently drops below the burst modethreshold, the burst mode controller 224 switches the inverter 102 backto burst mode operation.

During energy storage periods of burst mode operation, the DC-ACinverter 208 is driven such that no output current is generated by theinverter 102 (i.e., I_(B) is zero). During such periods, current isprecluded from flowing to the DC-AC inverter 208, thereby causingcurrent generated by the PV module 104 to charge the input capacitor220. The inverter 102 remains in an energy storage period for a numberof AC grid voltage cycles, N_(off), as determined by the burst modecontroller 224, before beginning a burst period of one AC grid voltagecycle. During the burst period, the burst mode controller 224 causes theDC voltage controller 210 to drive the DC-AC inverter 208 such that theburst current I_(B) is generated in accordance with the energy that hasbeen stored in the input capacitor 220 during the previous energystorage period. As a result, the inverter output current during a burstperiod, I_(B), is greater than the inverter output current duringcontinuous mode, I_(req), for a given level of solar irradiance andtemperature. In some embodiments, I_(B)=I_(req)*(N_(off)+1).

Due to the charging and discharging of the input capacitor 220 duringburst mode, a ripple voltage (“burst mode ripple voltage”, or V_(r))overrides the average voltage across the input capacitor 220, andresults in a corresponding burst mode ripple voltage overriding the PVmodule DC voltage, V_(DC). As such, the PV module operating voltagefluctuates in accordance with the magnitude of the burst mode ripplevoltage V_(r); the greater the burst mode ripple voltage V_(r), thegreater the PV module operating voltage excursion. Such a fluctuation inthe PV module operating voltage results in reduced efficiency of theinverter 102. For example, when operating proximate the MPP voltage, alarger voltage fluctuation around the MPP voltage results in a greaterperiod of time that the PV module 104 is operating off of the MPPvoltage, resulting in less than optimal power being drawn from the PVmodule 104.

The magnitude of the burst mode ripple voltage V_(r) varies inaccordance with the length of time the input capacitor 220 charges anddischarges; i.e., with the energy storage and burst periods. The burstmode controller 224 controls the energy storage period, i.e., the numberof AC grid voltage cycles of the energy storage period, N_(off), suchthat the burst mode ripple voltage V_(r) remains below a pre-determinedthreshold, thereby improving the efficiency of the inverter 102 duringburst mode operation.

When operating in continuous mode, the inverter 102 continuallygenerates output power in-phase with the AC grid power (i.e., duringeach AC grid voltage cycle). As such, the inverter output poweroscillates between zero output power at the AC grid voltagezero-crossings and peak output power at the AC grid voltage peakpositive and negative amplitudes. When the inverter output power is setat zero, current from the PV module 104 is precluded from flowing to theDC-AC inverter 208 and therefore charges the input capacitor 220. Whenthe inverter output power is set at peak, energy stored in the inputcapacitor 220 is utilized, in addition to the instantaneous power fromthe PV module 104, to generate a peak inverter output power of twice theaverage PV module output power. Thus, the charging and discharging ofthe input capacitor 220 during continuous mode provides an AC componentoverriding the average power provided by the PV module 104.

During both the continuous and burst modes of operation, the ripplevoltage caused by the charging and discharging of the input capacitor220 provides an opportunity for maximum power point tracking. As theripple voltage across the PV module 104 varies above and below anaverage (i.e., DC) PV module voltage, the PV module output power variesin accordance with the ripple voltage. If the PV module 104 producesmore power when operating above its DC voltage than when operating belowits DC voltage, then the PV module DC voltage is below the MPP and mustbe increased to reach the MPP. If the PV module 104 produces more powerwhen operating below its DC voltage than when operating above its DCvoltage, then the PV module DC voltage is above the MPP and must bedecreased to reach the MPP. Thus, the difference between the averagepower produced by the PV module 104 when it is operating above its DCvoltage and when it is operating below its DC voltage indicates in whichdirection the PV module DC voltage must be shifted to achieve the MPP.Additionally, if the difference is zero, the PV module 104 is biased atthe MPP. In some embodiments, such a power difference may be determinedbased on the PV module output power during certain phases of the AC gridvoltage, as further described below.

FIG. 3 is a block diagram of a burst mode controller 224 in accordancewith one or more embodiments of the present invention. The burst modecontroller 224 comprises a multiplier 302, an operating mode module 304,and a power measurement module 306.

The multiplier 302 receives signals indicative of the instantaneousinput current and voltage from the PV module 104, I_(in) and V_(in),respectively, through the I-V monitoring circuit 204, and generates anoutput signal indicative of the instantaneous input power from PV module104, P_(in). The output of the multiplier 302 is coupled to the powermeasurement module 306 and to the operating mode module 304;additionally, the power measurement module 306 receives inputs of I_(in)and V_(in). The operating mode module 304 is further coupled to the DCvoltage controller 210, and receives a signal indicative of an estimatedPV module input current, I_(est), from the DC voltage controller 210.The I_(est) is an estimated input current to be drawn from the PV module104 that will result in biasing the PV module 104 at a desired DCvoltage setpoint. The operating mode module 304 is also coupled to thepower measurement module 306, and additionally receives an input signalof a burst mode power threshold, P_(B).

The operating mode module 304 compares the PV module input power P_(in)to the burst mode threshold P_(B). If the inverter 102 is operating incontinuous mode and P_(in) is less that P_(B), the operating mode module304 switches the inverter 102 to burst mode; once P_(in) is greater thanP_(B), the operating mode module 304 switches the inverter 102 back tocontinuous mode.

The power measurement module 306 receives an input indicative of the ACgrid voltage phase from the conversion control module 214, for example,from a phase lock loop of the conversion control module 214. The powermeasurement module 306 integrates the PV module input power P_(in)during certain portions of the AC grid voltage cycle to obtain a firstpower “bin”, PB₁, and a second power “bin”, PB₂. Additionally, the PVmodule input voltage V_(in) is integrated over a portion of the AC gridvoltage cycle to obtain a DC voltage bin, V_(DC)B, where V_(DC)B isutilized to determine a DC (i.e., average) PV module input voltage,V_(DC). In some embodiments, during each cycle of the AC grid voltagewhen operating in continuous mode, the power measurement module 306integrates P_(in) during a 90°-180° phase of the AC grid voltage cycleto obtain PB₁, and integrates P_(in) during a 180°-270° phase of thesame AC grid voltage cycle to obtain PB₂. The PV module input voltageV_(in) is integrated over the entire 90°-270° phase to obtain V_(DC)B.During burst mode, the power bins and the average PV module inputvoltage are determined as described further below.

The power measurement module 306 determines a first and a second powermeasurement, P₁ and P₂, respectively, based on PB₁ and PB₂. Duringcontinuous mode, P₁=PB₁, and P₂=PB₂; during burst mode, the powermeasurements are determined as described further below. The first andsecond power measurements are supplied to the MPPT controller 212 fordetermining whether the PV module 104 is operating at, above, or belowthe MPP, and any required shift in the DC voltage setpoint to achievethe MPP. Additionally, the power measurement module 306 determines a PVmodule DC voltage, V_(DC), based on V_(DC)B. The power measurementmodule 306 supplies V_(DC) to the DC voltage controller 210 fordetermining I_(req). The new DC voltage setpoint and the required outputcurrent I_(req) are applied during the next AC grid voltage cycle.

When switching from continuous to burst mode, the operating mode module304 determines a maximum number of AC grid voltage cycles, N_(off), foran energy storage period such that the burst mode ripple voltage V_(r)will remain below a pre-determined threshold. In some embodiments, athreshold of 10% of the PV module DC voltage V_(DC) is utilized. In someembodiments, N_(off) is computed as follows:Noff<C*Vdc*Vr/Pin*T

In the above equation, C is the capacitance of the input capacitor 220,V_(DC) is the PV module DC voltage, V_(r) is the burst mode ripplevoltage, P_(in) is the input power from the PV module 104, and T is theAC grid voltage cycle period.

During burst mode, the power measurement module 306 integrates the PVmodule input power P_(in) over certain portions of the AC grid voltagecycle during an energy storage period to obtain PB₁ and PB₂. Todetermine the portions of the AC grid voltage cycle over which tointegrate P_(in), the N_(off) AC grid cycles of the energy storageperiod are partitioned into two equal portions—a “first half” of theenergy storage period and a “second half” of the energy storage period,where the first half occurs prior to the second half. For example, foran energy storage period of one AC grid voltage cycle, the first halfcomprises the AC grid voltage cycle from 0°-180°, and the second halfcomprises the same AC grid voltage cycle from 180°-360°. For an energystorage period of two AC grid voltage cycles, the first half comprisesthe entire first AC grid voltage cycle, and the second half comprisesthe entire second AC grid voltage cycle. For energy storage periodsgreater than two AC grid voltage cycles, the first and second halves areanalogously defined.

To obtain PB₁, the power measurement module 306 integrates the PV moduleinput power P_(in) during any of the 90°-270° AC grid voltage phaseperiods occurring within the first half of the energy storage period. Toobtain PB₂, the power measurement module 306 integrates the PV moduleinput power P_(in) during any of the 90°-270° AC grid voltage phaseperiods occurring within the second half of the energy storage period.Additionally, the power measurement module 306 integrates the PV moduleinput voltage V_(in) over each 90°-270° phase during the entire energystorage period, along with the 90°-270° phase during the subsequentburst period, to obtain V_(DC)B.

The measured power bins PB₁ and PB₂ are then used by the powermeasurement module 306 to determine the first and second powermeasurements, P₁ and P₂, respectively. In some embodiments,P₁=PB₁/N_(off) and P₂=PB₂/N_(off) during burst mode. Additionally, thepower measurement module 306 determines V_(DC) based on V_(DC)B. Thefirst and second power measurements P₁ and P₂ are supplied to the MPPTcontroller 212 and V_(DC) is supplied to the DC voltage controller 210for the appropriate MPPT and inverter output current processing. Theoutput of such processing, i.e., the new DC voltage setpoint and therequired output current from the inverter 102, are applied to the burstperiod following the next energy storage period.

In some instances, the input capacitor 220 may degrade over time anddegrade the performance of the inverter 102. In order to identify such acondition, the operation of the input capacitor 220 may be evaluatedduring burst mode when the number of AC grid voltage cycles in theenergy storage period (i.e., N_(off)) is two or more. In someembodiments, the power measurement module 306 determines a differencebetween the average input voltage V_(in) from the PV module 104 duringthe first and second AC grid voltage cycles of the energy storage period(i.e., ΔV). The capacitance C of the input capacitor 220 can then beestimated as follows:C=Iin/Fgrid*ΔV

Where I_(in) is the input current from the PV module 104, and F_(grid)is the frequency of the grid voltage. If the capacitance is below acapacitance threshold, the power measurement module 306 may provide anoutput alarm to indicate the condition.

FIG. 4 is a plurality of waveforms 400 depicting collection periods ofPV module power and voltage measurements during burst mode in accordancewith one or more embodiments of the present invention.

The waveform 402 corresponds to a computed energy storage period of oneAC grid voltage cycle. During a first AC grid voltage cycle T₁, aninverter is operating in continuous mode. The power bins PB₁ and PB₂ areobtained by integrating the PV module input power P_(in) during the90°-180° and 180°-270° phases of the AC grid voltage, respectively, andthe PV module input voltage V_(in) is integrated during the entire90°-270° period to obtain the DC voltage bin V_(DC)B. P₁ is set to PB₁,P₂ is set to PB₂, and the average DC voltage V_(DC) is computed fromV_(DC)B. The power and voltage measurements PB₁, PB₂, and V_(DC) areutilized to determine the inverter operating parameters for the next ACgrid voltage cycle in which the inverter generates output power.

At the end of T₁, the inverter switches to burst mode and enters anenergy storage period. The inverter generates no output power duringsuch a period. The power bin PB₁ is obtained by integrating the PVmodule input power P_(in) during the 90°-270° phase within the firsthalf of the energy storage period (i.e., the 90°-180° phase of T₂), andthe power bin PB₂ is obtained by integrating the PV module input powerP_(in) during the 90°-270° phase within the second half of the energystorage period (i.e., the 180°-270° phase of T₂). Additionally, the PVmodule input voltage V_(in) is integrated during the entire 90°-270°period of T₂ and to obtain the voltage bin V_(DC)B.

At T₃, the inverter begins a burst period and begins generating outputpower. The operating parameters determined during T₁ are adjustedaccordingly to drive the inverter to generate the burst mode current,I_(B). During T₃, the PV module input voltage V_(in) is integratedduring the entire 90°-270° period of T₃ and added to the voltage binV_(DC)B. P₁ is set to PB₁, and P₂ is set to PB₂, and V_(DC) is computedfrom V_(DC)B. The power and voltage measurements P₁, P₂, and V_(DC) areutilized to determine the inverter operating parameters for the next ACgrid voltage cycle in which the inverter generates output power.

At T₄, the inverter begins an energy storage period. Analogous to theoperation during T₂, PB₁, PB₂, and V_(DC)B are obtained. At time T₅, theinverter begins a burst period. The operating parameters determinedduring T₃ are adjusted accordingly to drive the inverter to generate theburst mode current, I_(B). Analogous to the operation during T₃, the PVmodule input voltage V_(in) is integrated during the entire 90°-270°period of T₅ and added to the voltage bin V_(DC)B. P₁ is set to PB₁, andP₂ is set to PB₂, and V_(DC) is computed from V_(DC)B. The power andvoltage measurements P₁, P₂, and V_(DC) are utilized to determine theinverter operating parameters for the next AC grid voltage cycle inwhich the inverter generates output power.

The waveform 404 corresponds to a computed energy storage period of twoAC grid voltage cycles. During a first AC grid voltage cycle T₁, aninverter is operating in continuous mode. The power bins PB₁ and PB₂ areobtained by integrating the PV module input power P_(in) during the90°-180° and 180°-270° phases of the AC grid voltage, respectively, andthe PV module input voltage V_(in) is integrated during the entire90°-270° period to obtain the DC voltage bin V_(DC)B. P₁ is set to PB₁,P₂ is set to PB₂, and the average DC voltage V_(DC) is computed fromV_(DC)B. The power and voltage measurements P₁, P₂, and V_(DC) areutilized to determine the inverter operating parameters for the next ACgrid voltage cycle in which the inverter generates output power.

At T₂, the inverter switches to burst mode and enters an energy storageperiod of two AC grid voltage cycles. The inverter generates no outputpower during such a period. The power bin PB₁ is obtained by integratingP_(in) during the portions of the 90°-270° phases occurring within thefirst half of the energy storage period (i.e., the 90°-270° phase ofT₂), and the power bin PB₂ is obtained by integrating P_(in) duringportions of the 90°-270° phases occurring within the second half of theenergy storage period (i.e., the 90°-270° phase of T₃). Additionally,the PV module input voltage V_(in) is integrated during each 90°-270°period of the energy storage period, i.e., the 90°-270° phase periods ofT₂ and T₃ to obtain the voltage bin V_(DC)B.

At T₄, the inverter begins a burst period and begins generating outputpower. The operating parameters determined during T₁ are adjustedaccordingly to drive the inverter to generate the burst mode current,I_(B). During T₄, the PV module input voltage V_(in) is integratedduring the entire 90°-270° period of T₄ and added to the voltage binV_(DC)B. P₁ is set to PB₁/2, P₂ is set to PB₂/2, and V_(DC) is computedfrom V_(DC)B. The power and voltage measurements P₁, P₂, and V_(DC) areutilized to determine the inverter operating parameters for the next ACgrid voltage cycle in which the inverter generates output power.

At T₅, the inverter begins an energy storage period. Analogous to theoperation during T₂ and T₃, PB₁, PB₂, and V_(DC)B are obtained. At timeT₇, the inverter begins a burst period. The operating parametersdetermined during T₄ are adjusted accordingly to drive the inverter togenerate the burst mode current, I_(B). Analogous to the operationduring T₂ and T₃, the PV module input voltage V_(in) is integratedduring the entire 90°-270° period of T₇ and added to the voltage binV_(DC)B. P₁ is set to PB₁/2, P₂ is set to PB₂/2, and V_(DC) is computedfrom V_(DC)B. The power and voltage measurements P₁, P₂, and V_(DC) areutilized to determine the inverter operating parameters for the next ACgrid voltage cycle in which the inverter generates output power.

The waveform 406 corresponds to a computed energy storage period ofthree AC grid voltage cycles. During a first AC grid voltage cycle T₁,an inverter is operating in continuous mode. The power bins PB₁ and PB₂are obtained by integrating the PV module input power P_(in) during the90°-180° and 180°-270° phases of the AC grid voltage, respectively, andthe PV module input voltage V_(in) is integrated during the entire90°-270° period to obtain the DC voltage bin V_(DC)B. P₁ is set to PB₁,P₂ is set to PB₂, and the average DC voltage V_(DC) is computed fromV_(DC)B. The power and voltage measurements P₁, P₂, and V_(DC) areutilized to determine the inverter operating parameters for the next ACgrid voltage cycle in which the inverter generates output power.

At T₂, the inverter switches to burst mode and enters an energy storageperiod of three AC grid voltage cycles. The inverter generates no outputpower during such a period. The power bin PB₁ is obtained by integratingP_(in) during the portions of the 90°-270° phases occurring within thefirst half of the energy storage period (i.e., the 90°-270° phase of T₂and the 90°-180° phase of T₃), and the power bin PB₂ is obtained byintegrating P_(in) during portions of the 90°-270° phases occurringwithin the second half of the energy storage period (i.e., the 180°-270°phase of T₃ and the 90°-270° phase of T₄). Additionally, the PV moduleinput voltage V_(in) is integrated during each 90°-270° period of theenergy storage period, i.e., the 90°-270° phase periods of T₂, T₃, andT₄ to obtain the voltage bin V_(DC)B.

At T₅, the inverter begins a burst period and begins generating outputpower. The operating parameters determined during T₁ are adjustedaccordingly to drive the inverter to generate the burst mode current,I_(B). During T₅, the PV module input voltage V_(in) is integratedduring the entire 90°-270° phase and added to the voltage bin V_(DC)B.P₁ is set to PB₁/3, P₂ is set to PB₂/3, and V_(DC) is computed fromV_(DC)B. The power and voltage measurements P₁, P₂, and V_(DC) areutilized to determine the inverter operating parameters for the next ACgrid voltage cycle in which the inverter generates output power.

FIG. 5 is a block diagram 500 of a DC voltage controller 210 inaccordance with one or more embodiments of the present invention. The DCvoltage controller 210 comprises an adder/subtractor 502, aproportional-integral (PI) controller 504, a gain module 506, and aburst mode adjustment module 508. The DC voltage controller 210 utilizesthe first feedback loop 216 to control the current supplied to the DC-ACinverter 208 such that the PV module 104 is biased at the DC voltagesetpoint.

The adder/subtractor 502 receives a signal indicative of the PV moduleDC voltage, V_(DC), from the burst mode controller 224, and a signalindicative of the DC voltage setpoint from the MPPT controller 212. Theoutput of the adder/subtractor 502 couples a difference between V_(DC)and the DC voltage setpoint to the PI controller 504. The PI controller504 acts to correct the difference by estimating an input current,I_(est), to be drawn from the PV module 104 that will result in biasingthe PV module 104 at the DC voltage setpoint.

The output of the PI controller 504 is coupled to the gain module 506and provides a signal indicative of I_(est) to the gain module 506. Thegain module 506 further receives an input of the AC grid voltageamplitude, V_(AC), from the conversion control module 214, and V_(DC)from the burst mode controller 224, and determines the required outputcurrent from the inverter, I_(req), to draw I_(est) from the PV module104. In some embodiments, I_(req) is calculated as follows:I _(req) =V _(DC) *I _(est) /V _(AC)*eff

In the above equation, eff is an efficiency rating of the inverter.

The output of the gain module 506 is coupled to the burst modeadjustment module 508. The burst mode adjustment module 508 adjusts therequired inverter output current I_(req) during burst mode such that itcorresponds to the burst mode output current, I_(B), in accordance witha control signal received from the burst mode controller 224. The outputof the burst mode adjustment module 508 is coupled to the DC-AC inverter208 and drives the DC-AC to generate an output current I_(req) duringcontinuous mode and an output current I_(B) during burst mode.

FIG. 6 is a block diagram of an MPPT controller 212 in accordance withone or more embodiments of the present invention. The MPPT controller212 comprises a power difference module 602, an integrator 604, and anadder 608. The MPPT controller 212 utilizes the second feedback loop 218to determine a DC voltage setpoint for the PV module 104 correspondingto the MPP voltage.

The power difference module 602 receives signals indicative of the firstand second power measurements, P₁ and P₂, from the burst mode controller224. The power difference module 602 computes a power difference betweenP₁ and P₂ and utilizes the power difference to determine an errorsignal, E. In some embodiments, the power difference is computed as(P₂−P₁)/(P₂+P₁).

The error signal ε from the power difference module 602 is coupled tothe integrator 604. The integrator 604 integrates the error signal ε andcouples the integrated error signal to the adder 608. The adder 608additionally receives an input of a nominal voltage, V_(nom), whereV_(nom) is an initial estimate of the MPP voltage. The integrated errorsignal acts to “fine tune” the nominal voltage such that the DC voltagesetpoint (i.e., the sum of the integrated error and the nominal voltage)corresponds to the MPP voltage. The DC voltage setpoint is then suppliedto the DC voltage controller 210 in order to drive the PV module 104 tooperate at the DC voltage setpoint.

FIG. 7 is a flow diagram of a method 700 for operating an inverter inburst mode in accordance with one or more embodiments of the presentinvention. In some embodiments, such as the embodiment described below,a power conversion device, such as a DC-AC inverter, is coupled to thePV module and converts DC power from the PV module to AC power, wheresuch AC power is coupled to a commercial power grid. In someembodiments, multiple PV modules may be coupled to a single centralizedDC-AC inverter; alternatively, individual PV modules may be coupled toindividual DC-AC inverters (e.g., one PV module per DC-AC inverter). Insome embodiments, a DC-DC converter may be coupled between the PV moduleor PV modules and the DC-AC inverter.

The method 700 begins at step 702 and proceeds to step 704. At step 704,the inverter operates in continuous mode and generates an outputcurrent, I_(req), such that the PV module is biased proximate the MPPvoltage. At step 708, a first and a second power measurement of theinput power from the PV module, P₁ and P₂, respectively, are obtained.In some embodiments, the first power measurement comprises integratingthe input power P_(in) from the PV module during a 90°-180° phase of anAC grid waveform cycle to obtain a first “power bin”, PB₁, where P₁=PB₁,and the second power measurement comprises integrating P_(in) during the180°-270° phase of the same AC grid waveform cycle to obtain a second“power bin”, PB₂, where P₂=PB₂. In some embodiments, P_(in) may besampled during such phases to obtain the first and second powermeasurements; for example, P_(in) may be sampled at a rate of 256 timesthe commercial power grid frequency. In alternative embodiments, thefirst and second power measurements may be obtained during differentphases of an AC grid waveform cycle.

Additionally, an average, or DC, voltage from the PV module, V_(DC), iscomputed. In some embodiments, the voltage V_(in) from the PV module isintegrated during the 90°-270° phase of the AC grid voltage to obtain aDC voltage “bin”, V_(DC)B. V_(DC) is then computed based on V_(DC)B.

The method 700 proceeds to step 710, where P₁, P₂, and V_(DC) areutilized to determine a DC voltage setpoint for the PV module such thatthe DC voltage setpoint corresponds to the MPP voltage. At step 712, arequired inverter output current, I_(req), that will result in the PVmodule being biased at the desired DC voltage setpoint is determined.Steps 704 through 712 of the method 700 comprise an outer feedback loopthat determines whether the PV module is currently biased at the MPPvoltage, and, if necessary, adjusts the DC voltage setpoint to achievethe MPP.

The method 700 proceeds to step 714, where a determination is madewhether the PV module input power exceeds a burst mode threshold. If thecondition at step 714 is met, the method 700 returns to step 704 and theinverter continues to operate in continuous mode. If the condition atstep 714 is not met, the method 700 proceeds to step 716.

At step 716, a maximum number of AC grid voltage cycles, N_(off), forenergy storage periods is determined based on a burst mode ripplevoltage threshold. In some embodiments, a burst mode ripple voltagethreshold of 10% of the PV module DC voltage V_(DC) is utilized. At step718, the inverter switches to burst mode, and begins an energy storageperiod of N_(off) AC voltage grid cycles. During the energy storageperiod, the inverter does not produce any output power, and powergenerated by the PV module is stored in the inverter.

At step 720, the first and second power bins, PB₁ and PB₂, arecollected, along with the DC voltage bin, V_(DC)B. In some embodiments,the N_(off) AC grid cycles are partitioned into two equal portions—a“first half” of the N_(off) AC voltage grid cycles and a “second half”of the N_(off) AC voltage grid cycles, where the first half occurs priorto the second half. PB₁ is obtained by integrating the PV module inputpower P_(in) during any of the 90°-270° AC grid voltage phase periodsoccurring within the first half, and PB₂ is obtained by integrating thePV module input power P_(in) during any of the 90°-270° AC grid voltagephase periods occurring within the second half. Additionally, the PVmodule input voltage V_(in) over each 90°-270° phase during the energystorage period to obtain V_(DC)B.

The method 700 proceeds to step 724. At step 724, following the energystorage period, a burst period is activated and the inverter begins togenerate output power. In some embodiments, the burst period comprises asingle AC grid voltage cycle. During the burst period, the requiredoutput current I_(req) determined in step 712 is adjusted such that theinverter generates a burst current, I_(B), in accordance with the amountof energy stored during the energy storage period. In some embodiments,I_(B)=I_(req)*(N_(off)+1).

At step 728, an input voltage from the PV module is measured and addedto the DC voltage bin V_(DC)B; in some embodiments, the PV module inputvoltage Vin is integrated over the 90°-270° phase of the AC grid voltagecycle and added to V_(DC)B. The method proceeds to step 730, where P₁,P₂, and V_(DC) are determined. In some embodiments, P₁=PB₁/N_(off),P₂=PB₂/N_(off), and V_(DC) is the average of V_(DC)B. At step 732, theDC voltage setpoint for the PV module is determined based on P₁, P₂, andV_(DC). At step 734, the required inverter output current, I_(req), thatwill result in the PV module being biased at the desired DC voltagesetpoint is determined.

The method 700 proceeds to step 736, where a determination is madewhether operation of the inverter should continue. If the condition atstep 736 is met, the method 700 returns to step 714; if the conditionsat step 736 is not met, the method 700 proceeds to step 738 where itends.

While the foregoing is directed to embodiments of the present invention,other and further embodiments of the invention may be devised withoutdeparting from the basic scope thereof, and the scope thereof isdetermined by the claims that follow.

The invention claimed is:
 1. A method for converting DC input power intoAC output power comprising: operating in a continuous mode, while the DCinput power is at a first level, wherein the DC input power iscontinuously converted into the AC output power and applied to an ACpower grid; and upon detecting the DC input power is at a second level,operating in a burst mode, wherein a burst of AC output power isfollowed by a period of no output power generation and an amount of ACoutput power for the burst is computed before applying the computedamount of AC output power to the AC power grid.
 2. The method of claim 1further comprising maximum power point tracking to substantiallymaintain the DC input power at a maximum amount during both thecontinuous and burst modes.
 3. The method of claim 1 wherein, during theburst mode, determining a number of AC grid voltage cycles, N_(off),that no output power is generated.
 4. The method of claim 3 whereinN_(off) is less thanC*V _(dc) *V _(r) /P _(in) *T where C is the capacitance of an energystorage capacitor; V_(dc) is an input voltage; V_(r) is a burst moderipple voltage; P_(in) is the DC input power; and T is an AC gridvoltage cycle period.
 5. The method of claim 3 wherein the amount of ACoutput power during the burst is a function N_(off).
 6. The method ofclaim 1 wherein, during the period of no output power, energy is storedin a storage device.
 7. The method of claim 6 wherein the storage deviceis a capacitor.
 8. The method of claim 7 further comprising minimizing aripple voltage across the capacitor.
 9. Apparatus for converting DCinput power into AC output power comprising: at least one controller forcausing the apparatus to operate in a continuous mode while the DC inputpower is at a first level and operate in a burst mode while the DC inputpower is at a second level; wherein, during the continuous mode, the DCinput power is continuously converted into the AC output power andapplied to an AC power grid and, during the burst mode, a burst of ACoutput power is followed by a period of no output power generation andan amount of AC output power for the burst is computed before applyingthe computed amount of AC output power to the AC power grid.
 10. Theapparatus of claim 9 further comprising a maximum power point trackingcontroller for substantially maintaining the DC input power at a maximumamount during both the continuous mode and the burst mode.
 11. Theapparatus of claim 9 further comprising an energy storage device forstoring energy during the period of no output power generation.
 12. Theapparatus of claim 11 wherein the energy storage device is a capacitor.13. A power generator comprising: a photovoltaic panel for generating aDC input power; and an inverter for converting the DC input power intoAC output power comprising at least one controller for causing theinverter to operate in a continuous mode while the DC input power is ata first level and operate in a burst mode while the DC input power is ata second level; wherein, during the continuous mode, the DC input poweris continuously converted into the AC output power and applied to an ACpower grid and, during the burst mode, a burst of AC output power isfollowed by a period of no output power generation and an amount of ACoutput power for the burst is computed before applying the computedamount of AC output power to the AC power grid.
 14. The power generatorof claim 13 further comprising a maximum power point tracking controllerfor substantially maintaining the DC input power at a maximum amountduring both the continuous mode and the burst mode.
 15. The powergenerator of claim 13 further comprising an energy storage device forstoring energy during the period of no output power generation.
 16. Thepower generator of claim 15 wherein the energy storage device is acapacitor.